Eat leading whitespace in link targets (wikitext, e8b3b86)

We do this during the sanitization phase because its only at that point that we know we've got valid input and are actually going to emit a link. If we did it during parsing but prior to confirming that we had a valid link then we might end up throwing away a space that we'll later need.

A follow-up link will make the same change for link encoding.

Signed-off-by: Greg Hurrell <greg@hurrell.net>